ENGINE WON'T RUN AT IDLE

I've just put a rebuilt engine in (2.5L non-turbo) and the car won't idle. The timing is good to the best of my knowledge (12 degrees), changed idle control valve, O2 sensor, map sensor checks out ok, coolant temp sensor checks out ok, battery is good, new coil, wires and plugs, compression is 130 plus or minus 5 pounds on all cylinders, vacuum is about 18 inches. Starts and runs ok untill it starts to warm up then it bucks a bit and stalls. Will run if I keep rpms at 2000 or above. Will run down to about 1500, but acts up. Oh yeah, no codes except for memory loss (12) from disconnecting battery neg to reset the computer. I've checked for vacuum leaks, but can't find any. I dropped the exhuast, but it made no difference. Also, before I changed the engine the old engine was running ok, but worn out (210,000miles). If I have to remove the intake it will mean pulling the engine because not enough room for torque wrench on re-install, so I'm try to get around that. I've also checked voltages and grounds from the computer and they appear to be good. I'm out of ideas.

Have done the fuel pressure and it is good 38-40psi, the engine replacement was a long block from S& J Engines in Spokane. Haven't sprayed the intake. It is interlaced with the exhaust manifold and was afraid of the excitement it might cause.

By acting up I mean it starts to miss. Sometimes I can keep it running by feathering the accelerator to get the idle up, but other times it just dies. Also, occassionally, like now, it won't start at all.
(when I dropped the exhaust before was when I had a no start condition, so after it still failed to start I just put it back together and eventually got it to start, but don't no why it started.)
I've just pulled the plugs and they were sooty as before during the no start condition, so I cleaned them, but still it won't start.

Ok. I got the car started, but again I don'know what I did to get it to start. I checked the voltages at the TSP and they were.874v to 3.84v (little low?)Fully closed to WOT and it went up smoothly. I checked the Auto shut down and fuel pump relays and they are operating ok. I tried to start it and it started.?
Now I'm back to the problem of it stalling when it starts to warm up. While I was checking for the fuel spray pattern as you suggested the car started to miss and I noticed that the timing light was cutting out and then reenergizing, so I think it could be ignition? Like I said I've changed the wires, coil, hall effect sensor. What else could it be that would cause it to fail when warm? Computer? I put in a new one (rebuilt) earlier on in this saga and it made no difference. Also, still no codes set in the computer.